Glucagon raises blood glucose only when which substrate is available in the body?

• A. Fat stores in the body
• B. Glycogen stores in the body
• C. Ketone bodies in the bloodstream
• D. Protein reserves in the body

Answer: (B)

Glucagon raises blood glucose by telling the liver to release glucose. The quickest and most direct way it does this is by stimulating glycogenolysis in the liver—that is, breaking down stored glycogen into glucose that can be released into the bloodstream. This relies on having glycogen stores available in the liver; without them, the immediate glucose-raising effect is limited, though the body can still make glucose over time via gluconeogenesis from noncarbohydrate substrates like amino acids, lactate, and glycerol. Keep in mind that glycogen stored in muscle can’t help raise blood glucose, because muscle glucose isn’t released into the bloodstream. Therefore, glucagon can effectively raise blood glucose only when liver glycogen stores are available.